Webel: Mathematica: CONVENTION: A 'type$' indicator for an '$opt$' or '$arg$' is for documentation only. It may use a "pattern helper", a human-friendly String, or a SysML-similar short-hand type representation. It need not correspond to a valid Pattern.
Webel: Mathematica: CONVENTION: Option help holders use a String variable with the prefix '$opt$', a related '$info$opt$' String, a 'def$opt$' default value expression, and optional '$warn$opt$', '$tip$opt$', '$lab$opt$', and 'type$opt$' indicator ...